jax.eval_shape(fun, *args, **kwargs)[source]#

Compute the shape/dtype of fun without any FLOPs.

This utility function is useful for performing shape inference. Its input/output behavior is defined by:

def eval_shape(fun, *args, **kwargs):
  out = fun(*args, **kwargs)
  shape_dtype_struct = lambda x: jax.ShapeDtypeStruct(x.shape, x.dtype)
  return jax.tree_util.tree_map(shape_dtype_struct, out)

But instead of applying fun directly, which might be expensive, it uses JAX’s abstract interpretation machinery to evaluate the shapes without doing any FLOPs.

Using eval_shape() can also catch shape errors, and will raise same shape errors as evaluating fun(*args, **kwargs).

  • fun (Callable) – The function whose output shape should be evaluated.

  • *args – a positional argument tuple of arrays, scalars, or (nested) standard Python containers (tuples, lists, dicts, namedtuples, i.e. pytrees) of those types. Since only the shape and dtype attributes are accessed, one can use jax.ShapeDtypeStruct or another container that duck-types as ndarrays (note however that duck-typed objects cannot be namedtuples because those are treated as standard Python containers).

  • **kwargs – a keyword argument dict of arrays, scalars, or (nested) standard Python containers (pytrees) of those types. As in args, array values need only be duck-typed to have shape and dtype attributes.


a nested PyTree containing jax.ShapeDtypeStruct objects as leaves.

Return type:


For example:

>>> import jax
>>> import jax.numpy as jnp
>>> f = lambda A, x: jnp.tanh(jnp.dot(A, x))
>>> A = jax.ShapeDtypeStruct((2000, 3000), jnp.float32)
>>> x = jax.ShapeDtypeStruct((3000, 1000), jnp.float32)
>>> out = jax.eval_shape(f, A, x)  # no FLOPs performed
>>> print(out.shape)
(2000, 1000)
>>> print(out.dtype)

All arguments passed via eval_shape() will be treated as dynamic; static arguments can be included via closure, for example using functools.partial():

>>> import jax
>>> from jax import lax
>>> from functools import partial
>>> import jax.numpy as jnp
>>> x = jax.ShapeDtypeStruct((1, 1, 28, 28), jnp.float32)
>>> kernel = jax.ShapeDtypeStruct((32, 1, 3, 3), jnp.float32)
>>> conv_same = partial(lax.conv_general_dilated, window_strides=(1, 1), padding="SAME")
>>> out = jax.eval_shape(conv_same, x, kernel)
>>> print(out.shape)
(1, 32, 28, 28)
>>> print(out.dtype)